Catalytic hydrodesulfurization is most typically performed using catalysts which initially consist of molybdenum (Mo.sup.+6) oxides dispersed on an alumina support. Catalytic activity is enhanced by addition of promoter elements, such as cobalt or nickel. The presence of a MoS.sub.2 (Mo.sup.+4) phase in the reduced and sulfided catalysts has been established by a variety of techniques, such as x-ray diffraction, laser Raman spectroscopy, EXAFS, see Parham, T. G. et al., J. Catal. 85, 295 (1984), and x-ray photoelectron spectroscopy. A catalytically active Co-Mo-S (or Ni-Mo-S) phase has also been identified for supported Co(Ni)Mo/Al.sub.2 O.sub.3 catalysts and unsupported MoS.sub.2 catalysts. This phase is thought to consist of cobalt (or nickel) atoms situated at the edges of MoS.sub.2 crystallites.
However, the complexity of the typical industrial supported Co(Ni)Mo/Al.sub.2 O.sub.3 materials and even unsupported MoS.sub.2 -based catalysts have made characterization of the catalytically important material difficult. In addition, the extent to which the molybdenum chemistry can be altered is limited because of the predominance of MoS.sub.2 for these catalysts.
Chevrel phase compounds are ternary molybdenum chalcogenides having a general formula M.sub.x Mo.sub.6 Z.sub.8, with Z being sulfur, selenium, or tellurium and with M being a metallic ternary component, see Chevrel et al. "Topics in Current Physics" (O. Fisher and Maple Eds.), Vol. 34, p. 25. Springer, Berlin, 1982 also Vol. 34, p. 87. These materials have been found to be active for thiophene desulfurization at 400.degree. C. under steady state conditions. Their hydrogenation activity for 1-butene is relatively low, however, compared to conventional cobalt molybdate catalysts. The discovery of the activity and selectivity of the Chevrel phases is important to catalytic hydrodesulfurization and doubly important since it has been found that Chevrel phase compounds have lower hydrogenation activities than comparable MoS.sub.2 catalysts.
Chevrel phase catalysts are structurally very different from conventional MoS.sub.2 -based catalysts. The anisotropic layer structure of MoS.sub.2 involves trigonal prismatic coordination of the molybdenum atoms by six sulfur atoms; bonding is strong within layers, while only weak interactions exist between the layers. In contrast, the Chevrel phases exhibit a much different molybdenum structural chemistry. As metal-rich materials, the Chevrel phases are psuedomolecular compounds based on a Mo.sub.6 S.sub.8 cluster. These Mo.sub.6 S.sub.8 units exist as distorted molybdenum octahedra having apexes which lie slightly outside the face centers of a distorted cube of sulfur atoms; the Mo.sub.6 S.sub.8 clusters are interconnected by Mo-S and Mo-Mo bonds.
Nearly forty metals can function as the ternary component for the Chevrel phases. The MoS.sub.2 layer structure only permits intercalation of some alkali and alkaline earth elements. The location of catalytically important transition metals such as cobalt and nickel in catalysts based on MoS.sub.2 is still uncertain. For the Chevrel phases, the location of the ternary metal "promoter" is much less ambiguous. The arrangement of Mo.sub.6 S.sub.8 clusters in the lattice gives rise to infinite channels running parallel to the rhombohedral axis, and the ternary component atoms are located at specific crystallographic sites in these channels. The structure allows the accommodation of ternary component cations of "large" size (e.g. Ho, Pb, and Sn), "intermediate" size (e.g. Ag and In), and "small" size (e.g. Cu, Fe, Ni, Co).
One of the most important aspects of molybdenum catalyst chemistry is the molybdenum oxidation state. For conventional hydrodesulfurization catalysts, the dominant oxidation state is +4 due to the present of MoS.sub.2. In contrast, the Chevrel phase materials have formed oxidation states between +2 and +22/3, depending on the concentration and/or valence of the ternary component.